1. Field of the Invention
The present invention relates to a memory system having a hot-swap function and, more particularly, to a memory system having serially-connected memory modules, capable of performing a hot-swap operation that replaces a failed memory module by a backup memory module without stopping operation of the memory system. The present invention also relates to a hot-swap method in such a memory system.
2. Description of the Related Art
There is known an information processing system having a plurality of memories (memory modules) connected to each other in a serial connection to allow a memory controller to control the data writing/reading operations. When a failure occurs in one memory module in such an information processing system, the entire system must be stopped in order to replace the failed memory module with another memory module, in general. This is because interruption of an access to the memory system makes it impossible to allow the information processing system to continue the processing operation.
As a method that replaces a failed memory module without stopping operation of the information processing system, a mirroring technique is known in which the bus has a redundancy configuration. FIG. 13 shows this mirroring technique. In the mirroring technique, there is provided a normal bus 102 to which memory modules 104, 105 and 106 in service are connected, as well as a backup bus 103 having a configuration similar to the normal bus 102, to which memory modules 107, 108 and 109 having a configuration similar to that of the memory modules 104, 105 and 106 are connected. If a failure occurs in any of the memory modules 104, 105 and 106, a memory controller 101 firstly recreates the data of the failed memory module from redundant data of other normal memory modules and migrates the data of the respective memory modules including the recreated data, to the corresponding backup memory modules. Then, the memory controller 101 stops operation of the normal bus 102 and switches to the backup bus 103 so as to perform a subsequent processing. After replacement of the failed memory module has been completed, the memory controller 101 switches from the backup bus 103 to the normal bus 102 to perform a normal processing. There is substantially no possibility of occurring of an error in the mirroring technique, and it is regarded as a reliable method. However, it requires memory modules twice as much a capacity as memory modules in service have, thereby resulting in an increase of the cost.
FIG. 14 shows a memory system having a plurality of memory modules, in which a single backup memory module 15 is provided for normal memory modules 11 to 14. When a failure occurs in any of the memory modules 12 to 14, an access from a memory controller 10 is switched from the failed memory module to the backup memory module 15. In this method, although it is possible to stop the failed memory module while the system is being run, replacement of a memory module, i.e., removal of the failed memory module and introduction of a new memory module in service cannot be performed without stopping the entire processing system. In this case, the backup memory module 15 can be turned on/off using a switch. However, when the switch is operated while the memory access is being executed, a transmission waveform may be adversely affected by the switching, thereby preventing a stable operation of the processing system.
In recent years, a memory system in which a plurality of serially-connected memory modules are used to perform a serial transmission has been put in a practical use. FIG. 15 shows an example of such a memory system using a conventional serial transmission scheme. As shown in FIG. 15, buffers 22 to 24 are provided for respective memory modules 19 to 21, and the transmission of a signal from a memory controller 25 is controlled in one direction by using a dedicated write bus 18 and a dedicated read bus 17. In this memory system, when power supply for a specific memory module is stopped, signal transmission to the next memory module is disabled. Thus, in the conventional memory system using a serial transmission scheme, it is impossible to disconnect the memory module without stopping operation of the processing system.
Patent Publication JP-2004-185199A (FIG. 13) describes a memory system using a serial transmission scheme capable of performing a hot-swap operation. When a failure occurs in this memory system, data is migrated to a hard disk drive and then switching from a serial transmission line to a bi-directional transmission line is performed, thereby allowing the disconnection of a failed memory module without interruption of power supply.
In the memory system described in JP-2004-185199A, data is migrated to a hard disk drive to allow the disconnection of a failed memory module. However, since the access speed of a memory and that of a hard disk differ from each other, it is impossible to allow the memory system to operate in a normal mode because the data is stored on the hard disk having a lower access speed.